The underlying invention generally relates to the field of modulator architectures of digital communication devices which are used for the processing of modulated RF signals, especially to a power amplifier circuit of a transmitting block for controlling the output power of an RF signal to be transmitted.
The wireless communications area has experienced a remarkable growth, and, as a result, the demand for high-efficient power control circuitries applied to wireless communications devices has increased dramatically. Because in a portable wireless environment all circuits are drawing power from a small battery, one of the most important aspects of circuitdesign. is to optimize the power consumption. Additionally, as wireless communication devices must be realized as low-cost products, the cost of the applied circuits must be reduced as well.
The basic structure of a conventional wireless communication system consisting of a QAM transmitter and a QAM receiver, respectively, is depicted in FIGS. 1a+b. On the transmitter side 100a of the system, which is shown in FIG. 1a, the basic operations are as follows: The digital data is first encoded (ik, qk), and then, after being submitted to an digital-to-analog converter 102, the complex-valued orthogonal in-phase (I) and quadrature (Q) channels are combined by means of a quadrature modulator 104. The resulting signal combination is up-converted to the RF carrier frequency by means of an internal mixing stage. Then, after having passed a filtering stage, the RF signal drives a power amplifier 106, whose output signal x(t) is fed to a transmission (TX) antenna 108. The antenna radiates the signal into the air, and the transmission is complete. The receiver side 100b, which is depicted in FIG. 1b, is simply the inverse, although slightly different components have to be used. First, the received RF signal x(t) is filtered to select the RF band of interest. After that, it is fed to a low noise amplifier 112 (LNA). The signal is then usually filtered and either directly down-converted from the passband to the baseband (in case of a homodyne receiver) or mixed to one or more intermediate frequency (IF) stages (in case a heterodyne or super-heterodyne receiver is employed). Thereby, the final mixing stage separates the signal into its I and Q components. Once at baseband, the I and Q signals are submitted to an analog-to-digital (A/D) converter 120 before they are further processed.
The power amplifier (PA) is the component of the system that takes the signal to be transmitted and amplifies it to the necessary level needed to drive the antenna for a particular power output level. In most wireless communications systems, the PA is the largest power consumer, usually because the amount of power that needs to be sent to the antenna (the power output) is itself very large. This does not include the total power that is consumed within the PA, just the amount that is required to drive the antenna. The total power consumed by the PA is necessarily greater than the power output, as there will always be some power consumed in the active devices and the peripheral circuitry. Since the power output specification itself is often larger than the power consumption of the rest of the blocks in the RF system and the power consumption of such a PA will be higher than the specified power output, the PA is decidedly the major power consumer of the system.
The signal to be transmitted is often transmitted by applying the output of the PA to a load device, be it a real circuit element or an antenna or similar device. Because the levels of power required to reliably transmit the signal are often quite high, there is a lot of power consumed within the PA. In many wireless applications, the amount of power consumed by this amplifier is not critical; as long as the signal being transmitted is of adequate power, that is good enough. However, in a situation where there is only a limited amount of energy available, which is not sufficient for the transmission procedure, the power consumed by all devices must be minimized, so as to maximize the length of time for which that energy is available.
The number of different classes of power amplifiers which are used today is too numerous to be counted, and they range from entirely linear to entirely non-linear, as well as from quite simple to inordinately complex. In PA terminology, a “linear” power amplifier is one which has a linear relationship between its input and output. Although a PA may comprise transistors operating in a nonlinear fashion (e.g. in case a FET switches between cutoff and saturation), it can still be considered linear. While nonlinear PAs feature a comparatively high efficiency, their nonlinearity causes the output signal to spread (due to intermodulation products, especially if there is a lot of phase noise in the local oscillator which will cause spreading of the input to the PA).
A power amplifier can consist of several serial stages. Each stage is usually more powerful than the previous one. As most of the quiescent current is drawn by the high power stages, which are not required for the low output power levels needed for wireless communication, means for bypassing high power stages when they are not required can lead to a significant reduction of energy consumption.
Since wireless telephones operate on battery power, it is also desirable that their transmitters operate as efficiently as possible to conserve power and extend battery life. Ideally for W-CDMA systems, such as those governed by the UMTS standard, power amplifier stages should be capable of efficient, linear operation in their required dynamic range. However, the prior art has not yet come close to the ideal, and many wireless telephones are having poor power management now. During low power transmissions, power is wasted by cascaded amplifier stages that are not needed. Consequently, attempts have been made to bypass unused stages.
Under normal operating conditions, conventional wireless transceivers devices use an APC circuit to control the output power of their amplification stages. The APC circuit found in most RF transceivers has an external connection that is intended to be connected to a linear power amplifier. After having detected the power of the modulated RF signal at the output of the final power amplifier, said signal is converted to a DC voltage and fed back to a variable-gain intermediate frequency (IF) stage in order to keep the final output power constant over a long period of time. As the APC voltage generation is done very early, the gain drift, which is caused by thermal drift, operating voltage deviation, etc., is not compensated by the circuit. Another option is to derive the ALC voltage from the drive power of the final amplifier and feed it to the external APC input of the RF transceiver. The theory is that when the power amplifier becomes overdriven, it will produce a negative voltage that is fed back into the transceiver's APC circuitry. This voltage acts as a gain control in the transmit stages of the transceiver which, in turn, automatically lowers the drive power (the transceivers output power) and limits distortion from the overdriven amplifier.
FIG. 2a shows a schematic block diagram for a conventional automatic power control (APC) loop according to the state of the art, which is used for stabilizing the signal level at the output port of an analog circuitry realizing an RF signal generator. Thereby, said circuit can also be used for executing an amplitude modulation. It comprises an frequency synthesizing unit (FSU), a power divider (e.g. a directional coupler), which feeds the reflected wave of the modulated RF output signal to a wideband detector diode, and an amplification stage whose output signal is fed to an electronically controllable attenuator (e.g. an amplitude modulator stage realized by means of PIN diodes). In case said RF signal generator is used for sweep-frequency applications, an external detector is usually applied in order to keep the signal level at the input port of a tested RF unit constant. Thereby, it should be noted that the output impedance of the FSU is changed due to the APC loop.